Dispersive spectrometry installations having single channel detection have been known for a long time.
In general, this type of installation comprises:
an inlet slit illuminated by a beam of electromagnetic radiation to be analyzed;
a dispersive stage having an outlet slit for delivering a dispersed spectral image of said beam, the image being limited to a selected band of the spectrum;
a single channel detection module receiving said spectral image via said outlet slit; and
processor means for analyzing the signals received by the single-channel detection module.
In practice, the single-channel detection module comprises photoelectric detector components such as photocells, photomultipliers, or thermopiles.
In operation, a single spectral component of the electromagnetic radiation to be analyzed passes through the outlet slit and is consequently detected by the single-channel detection module. As a result, the dispersive component needs to be displaced to cause each of the other spectral components to pass through the outlet slit.
Now that integrated detector components such as photodiode strips and charge transfer devices are available and can advantageously be coupled with image intensifier systems, single-channel detection is often replaced by multi-channel detection, since with multi-channel detection the dispersed spectral image can be projected directly onto the multi-channel detection module, i.e. without passing through an outlet slit, thereby enabling a plurality of spectral components of the electromagnetic radiation to be detected simultaneously without altering the configuration of the installation.
However, signal acquisition for obtaining the spectrum of the electromagnetic radiation to be analyzed takes place when the dispersed spectral image of said electromagnetic radiation is stationary relative to the multi-channel detection module, thereby limiting the width of the spectrum to the geometrical dimensions of the detection module and limiting the spectral resolution of the installation to that of the detector components.
In addition, when an image intensifier system is coupled with the detector components, successive spectra are always observed using the same light-emitting phosphors. As a result the speed of said phosphors determines the time resolution of the installation.
Furthermore, the level of interfering light due to light being diffused by the dispersive component or by the other optical components is higher than in single-channel detection installations. With single-channel detection, this level is proportional to the area of the inlet and outlet slits which are generally chosen to be very narrow (e.g. a few tens of micrometers across). With multi-channel detection, the level of interfering light is proportional to the area of the detection module (of the order of a few square centimeters) given that the dispersed spectral image from the dispersive stage is projected directly onto the detection module (there is no outlet slit with multi-channel detection).
Finally, the signal-to-background noise ratio of the installation is unsatisfactory, in particular because of noise due to the detection module itself, noise induced by external disturbances, and photon noise generated by the detection module being illuminated by a continuous background of interfering light or by interfering spectral bands.